According to the present electrokinetic theory, all electrokinetic phenomenon (e.g., streaming potential, electrophoresis, sedimentation potentials, and electro-osmosis) are interrelated phenomenon and are based upon the fundamental electrochemical concept of the "double layer". The double layer concept was first described by Helmholtz in the last half of the nineteenth century and has been modified by many others since. In essence, an electrical double layer surrounds any surface in contact with a liquid and consists generally of an immobile layer of ions next to the surface and a mobile layer of ions electrostatically in equilibrium with the ions in the immobile layer. The Encyclopedia of Electrochemistry edited by Hampell (Reinhold Publishing Corp. 1964) identified the double layer as follows.
"At any phase boundary there is always some redistribution of electrical charge because of the inhomogeneous field. This may be represented as two parallel sheets of charge of opposite sign known as a double layer. This name is retained even if the structure is more complex."
Clarles Reilley in his article entitled, "Fundamentals of Electrode Processes", Treatise on Analytical Chemistry, edited by Kolthoff and Elving (John Wiley and Sons, Inc. 1963), Part I, Volume 4, Chapter 42, at page 2127-2129, discusses in greater detail the electrical double layer. In particular, the double layer is analogized to the assemblage of a plurality of charged layers at the liquid-solid interface. Unfortunately, there is only a modicum amount of literature which discusses the electrokinetic effects at the double layer. Nevertheless, the principal difficulty of analyzing the double layer is well documented. See e.g., Bockris and Reddy, "Modern Electrochemistry", page 644, Volume 2 (Plenum Press 1974). Basically, the problem is that the introduction of an electrode into the liquid electrolyte sets up a second double layer. Thus, what is measured is the potential difference across two double layers in series and not just the double layer of interest.
The electrokinetic effects caused by changes in the double layer, if these effects could be accurately measured, have numerous practical applications. One application is in the mineral processing technology and particularly in the processing of low grade ores and ultrafine mineral particulates. Mineral processing involves many unit operations dealing with dispersions of a solid in a liquid where the dispersions range from course particles mixed with water to microscopic particles of colloidal size in a variety of liquid media. Separation of the desired mineral can be done by at least three general processes: sedimentation (e.g., decantation, flocculation, centrifugation, clarification, thickening, froth floatation, and electroflotation), filtration, (e.g., ultrafiltration, microfiltration, and expression) and electrical separation (electrophoresis, electrodialysis, electrodecantation, and electrochromatography, for example).
Each of the above processes involves a different primary mechanism for separation whereby sedimentation is a gravitational phenomenon, filtration is a pressure phenomenon, and electrical separation is an electrical phenomenon. However, each of the processes has the common factor that effects the degree of separation achieved, namely the surface charge of the particles.
Particle surface charge is generally assumed to be due to the establishment of a so-called ionic double layer on the surface of the particle. This double layer is believed to consist of ions and liquid molecules bound to the surface of the solid and counter ions, in the liquid near the surface, electrostatically attached to the bound ions. In essence, a solid particle immersed in a liquid, establishes a charge separation between itself and the bulk of the liquid. This charge separation in turn establishes a potential gradient and by physical and chemical manipulation of this potential gradient, it is possible to significantly improve the efficiency of the aforementioned processes. Consequently, in order to achieve maximum efficiency of the processes, it is important to be able to continuously measure the effects of the double layer potential (it being physically impossible to measure the double layer potential per se, as discussed above).
Unfortunately, all of the known conventional methods of measuring the effects of the double layer potential can be done only in a batch mode and not continuously. Some of the prior art electrokinetic techniques for measuring the effects of the double layer potential involve investigating streaming potential, electrophoretic mobility, and sedimentation potential. But there is no presently known way of investigating these effects under factory conditions on a continuous basis.
Present systems used to monitor the double layer effects and to maintain the surface of the particular mineral at the desired charge utilize pH and conductivity sensors. Such sensors however, do not measure surface charge but rather, based on prior experience and laboratory tests, the desired surface charge can be maintained by adjusting the pH and/or the conductivity of the dispersion. As a result, such a method does not provide real time information on the magnitude and polarity and the surface charge. In addition, in many of the aforementioned mineral processing operations, control of the surface charge of the dispersion cannot be done on a scientific basis.
Conventional sensors which can be used on a real time basis are effectively limited to pH sensors and conductivity sensors. Many of these sensors can be used to sample either the entire dispersion as it is being flowed through the processes or a sample of the dispersion. Typical conductivity cells are disclosed in the U.S. Patents to Hood No. 2,870,078; to Adelson No. 2,768,135; and to Marks, No. 2,382,735. Although these references disclose conductivity cells that are useable in monitoring some of the above processes, they do not provide direct information about the double layer or about the surface charge.